wherry: determining soil acidity 219 



with reference to that of water as a unit.^ Correspondingly, 

 specific alkahnity is the amount of alkali, as measured by the 

 hydroxyl ion, the unit being the same. Specific acidity and alk- 

 alinity numbers can be readily transposed into P^ values by 

 anyone preferring that mode of statement. Find the power of 

 10 corresponding to the number; if acid, subtract from 7; if 

 alkaline, add 7 ; the result is the P„. 



Under the most favorable conditions it is possible by the indi- 

 cator method to measure acidity and alkalinity with much 

 greater precision than is here attempted. By treating the indi- 

 cators with buffer solutions of known ionic concentration, many 

 hues intermediate between those here tabulated can be dis- 

 tinguished. On comparing the colors thus produced with those 

 developed by mixing clarified soil extracts with the same indi- 

 cators, specific acidities differing by a factor of Vio or 1.59 

 (Ph = 0.2) can be recognized. In the field, where it is incon- 

 venient to carry buffer solutions to prepare standards for com- 

 parison, and where the turbidity of soil extracts is difficult to 

 remove, it is impracticable to work closer than values differing 

 by a factor of Vio or 3.16 (Fh = o-5) which is rounded off for 

 simplicity to 3 -f . This degree of precision is, however, entirely 

 adequate for the purpose in view, for it has been repeatedly 

 found that from one to another plant of the same species, or 

 indeed, from one to another root on the same individual, sep- 

 arate observations of reaction may differ by a factor of 10 or 

 more. To give a specific example, a plant of Rhododendron 

 maximum growing in glacial drift near Williamsport, Pennsyl- 

 vania, was found to have some of its roots in soil with a specific 

 acidity of i (neutral), and other roots in soils with acidities of 

 3, 10 and 30. It seems obvious that nothing would be gained 

 by measuring the acidity on any one of these to a high degree 

 of precision, when the soils around the plant as a whole varied 

 by a factor of 30. 



* Only acidity in this sense is considered here; the effect variously termed latent, 

 potential or negative acidity, and often shrouded in mystery by writers who fail 

 to appreciate the significance of adsorption and other physical-chemical phenomena, 

 has no bearing on the problem in hand. 



